Paper machine

ABSTRACT

A paper machine for stable high-speed papermaking without slackness of wet web, the paper machine including the most downstream press unit along the transfer path of the wet web in the press part, and the most upstream dryer unit and the second upstream dryer unit along the transfer path of the wet web in the dryer part are associated with three different belt mechanisms, respectively, so that the driving units of the three driving units associated with the most downstream press unit and the first and second upstream dryer units are individually controlled so that a transfer speed of the wet web along each of the most downstream press unit and the first and second upstream dryer units is individually set.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a paper machine for papermaking and particularly to a structure of a press part for pressing a wet web formed in a wire part to dewater the wet web and a dryer part for drying the wet web dewatered in the press part.

[0003] 2. Description of the Related Art

[0004] In line with recent increased demand for a high-speed paper machine, there is proposed a no-open-draw paper machine in which a wet web is transferred from the former to the press unit, always being supported by a press felt, an impervious transfer belt, or a dryer canvas (a DRY felt). A no-open-draw paper machine is a paper machine having no open draw, that is, a state in which a wet web is never transferred in the air freely without being supported. The no-open-draw structure can minimize air resistance to a wet web during transfer so that the wet web can be sufficiently protected from tearing even when a paper machine is working at high speed.

[0005] Such a no-open-draw paper machine is disclosed in, for example, publications of United States Patent Number (hereinafter abbreviated to USPN) U.S. Pat. No. 4,483,745, U.S. Pat. No. 5,792,320, U.S. Pat. No. 5,951,821. FIG. 3 schematically shows a side view between the last press and the first dryer group of the paper machine appearing in publications of U.S. Pat. No. 5,951,821 and U.S. Pat. No. 5,792,320. A web formed by a non-illustrated former is pressed by a press 101 upstream to be dewatered and then sent to the last press 102.

[0006] At the top side of the last press 102, a top felt belt 102 a made of water-absorbing felt runs around a suction pick-up roll 102 b and a top press roll 102 c. Meanwhile, an impervious belt 102 d made of an impervious material runs around a bottom roll 102 e and other rolls.

[0007] In the paper machine with such a structure, a wet web 120 sent from the upstream presses 101 is sucked by the suction pick-up roll 102 b to adhere to the top felt belt 102 a and then transferred by the top felt belt 102 a to be introduced to the last press 102. Both surfaces of the wet web 120 are pressed through the nip between the top press roll 102 c and the bottom press roll 102 e to be dewatered, and water drained off from the wet web 120 is absorbed by the top felt belt 102 a.

[0008] A dryer part 103 is installed downstream of the last press 102. In the dryer part 103, a wet web 120, which has been transferred on the impervious belt 102 d, is sucked by a suction pick-up roll 103 b thereby clinging to a dryer canvas 103 a. A suction box 104 arranged at the inside of the loop-shaped canvas 103 a sucks the wet web 120, which clings to the canvas 103 a. In this state, the canvas 103 a transfers the wet web 120 to a first dryer cylinder 103 e. After passing the first dryer cylinder 103 e, the wet web 120 is turned by a turning roll 103 f and then transferred to a second dryer cylinder 103 g. In the same manner, the wet web is alternately transferred downstream along a dryer cylinder and a turning roll, which do not appear in the drawing. While the wet web 120 is transferred along the dryer cylinders and the turning rolls, the wet web 120 gradually becomes dried by being pressed by the dryer canvas 103 a.

[0009] Application of pressure to the wet web 120 in the press part including the last press 102 in order to dewater the wet web 120 reduces the thickness of the wet web 120 and, at the same time, slightly extends the wet web 120 in the direction (e.g., the transfer direction of the wet web 120) perpendicular to that of the pressure application. Such extension is generated on the wet web 120 each time pressure is applied to cause an accumulated elongation of the wet web 120. As a result, the wet web 120 becomes more elongated as the wet web 120 is transferred downstream in the transfer direction of the wet web 120 whereupon the tension of the wet web 120 gradually decreases.

[0010] As a solution, transfer speed of the suction pick-up roll 103 b is set to be higher than the transfer speed of the wet web 120 from the last press 102 so that the wet web 120 is drawn between the suction pick-up roll 103 b and the last press 102 in order to absorb the elongation of the wet web 120.

[0011] In the paper machine of FIG. 3, elongation (slackness) of the wet web 120 on the dryer canvas is mainly classified into first longitudinal elongation caused when the dryer canvas 103 a disengages from the first dryer cylinder 103 e and second longitudinal elongation caused when the wet web 120 is dried by contacting with the dryer cylinders (especially, the second dryer cylinder). Hereinafter, the first and second longitudinal elongation will be described respectively.

[0012] At first, the first longitudinal elongation will now be described in conjunction with a system for removing water from the wet web 120 at the dryer part 103.

[0013] The wet web 120 received by the suction pick-up roll 103 b is sent to downstream dryer cylinders 103 e, 103 g . . . in succession to be pressed onto the outer surfaces of the dryer cylinders 103 e, 103 g . . . by the dryer canvas 103 a. At that time, the outer surfaces of the dryer cylinders 103 e, 103 g . . . are smooth enough to allow the wet web 10 to cling to the outer surfaces because of the presence of water, paper dust and sticky materials (hereinafter called water and so forth) on the surface of the wet web 120.

[0014] When the wet web 120 clinging to the outer surface of a dryer cylinder is about to disengage from the outer surface, a part of the water and so forth on the surface of the wet web 120 remains on the outer surface of the dryer cylinder. The remaining water and so forth on a portion of the outer surface is removed from the surface by being scraped off by a doctor blade or the like or by being dried until the portion comes into contact with the wet web 120 again due to the rotation of the dryer cylinder (namely, the amount of the removed water and so forth is substantially identical to that of the remaining water and so forth whereby the outer surface of the dryer cylinder is in an equilibrium state).

[0015] As mentioned above, the wet web 120 clings to the outer of the dryer cylinders 103 e, 103 g . . . . Above all, since a larger amount of water and so forth adheres to the outer surface of the first dryer cylinder 103 e, the wet web 120 tends to cling to the outer surface of the first dryer cylinder 103 e with ease.

[0016] For this reason, even after the wet web 120 clinging to the first dryer cylinder 103 e passes a separation point of the dryer canvas 103 a from the first dryer cylinder 103 e and is no longer pressed to the first dryer cylinder 103 e by the dryer canvas 103 a, the adhesive force of the wet web 120 overcomes the centrifugal force thereof and the wet web 120 adheres to and is transferred together with the outer surface of the first dryer cylinder 103 e as the broken line of FIG. 3 shows. When a portion of the wet web 120 is transferred adhering to the outer of the cylinder surface, the tension of the portion gradually increases to disengage the wet web 120 from the first dryer cylinder 103 e. However, at this time, the wet web 120 generates elongation, that is first longitudinal elongation.

[0017] A pressure difference generating apparatus (exemplified by a suction box) 105 is installed near to the separation point A of the first dryer cylinder 103 e, facing the wet web interposed by the dryer canvas 103 a. The pressure difference generating apparatus 105 sucks the wet web 120 onto the dryer canvas 103 a so that the wet web 120 is inhibited from adhering to the first dryer cylinder 103 e after the separation point A.

[0018] In order to assist the pressure difference generating apparatus 105, a paper-separation doctor 106 and an air jet 106 a are disposed downstream of the separation point A in the rotational direction of the cylinder so as to face the first dryer cylinder 103 e. The tip of the paper-separation doctor 106 and air jetted from the air jet 106 a extricate the wet web 120 adhered to the outer cylinder surface from the surface. The turning roll 103 f to guide the wet web 120 from the first dryer cylinder 103 e to the second dryer cylinder 103 g generates a negative pressure around the surface thereof so that the centrifugal force of the wet web 120 running around the turning roll 103 f does not disengage the wet web 120 from the turning roll 103 f, thereby avoiding attachment of the wet web 120 to the first dryer cylinder 103 e.

[0019] It is sure that such a structure reduces first longitudinal elongation, but it is impossible to inhibit first longitudinal elongation completely.

[0020] As the wet web 120 is drying as the wet web 120 proceeds through the dryer part, progressing reductions in viscosity of the water on the wet-web surface and in amount of the sticky materials cause a sudden decline of the adhesive force of the wet web 120 to the outer surface of a dryer cylinder. As a result, at the second and its downstream dryer cylinders, the wet web 120 generates slight first longitudinal elongation due to adhering to the outer surfaces of these cylinders.

[0021] Next, the second longitudinal elongation is caused by expansion of water contained in the wet web 120, which water is heated as the wet web 120 is transferred in contact with the dryer cylinders 103 e and 103 g. Generally, such longitudinal elongation is generated to a larger degree at the second dryer cylinder 103 g and downstream dryer cylinders thereof than at the first dryer cylinder 103 e. This is because the wet web 120 being transferred along the first dryer cylinder 103 e is not heated to so high a temperature that the water contained in the wet web 120 does not expand very much while the wet web 120 being transferred along the second and downstream dryer cylinders 103 g . . . is heated to so high a temperature that the water in the wet web 120 expands a lot. The longitudinal elongation caused by expansion of contained water is called the second longitudinal elongation, which is largely generated on the wet web 120 when transferred along the second dryer cylinder 103 g while the first longitudinal elongation is generated mainly at the first dryer cylinder 103 e.

[0022] When the wet web 120 has passed only several dryer cylinders, the wet web 120 generates such first and second longitudinal elongation and thereby goes slack on the dryer canvas 103 a. If a slack portion of the wet web 120 is pressed onto a dryer cylinder, creases are made on the portion of the wet web 120 and stress concentration on a folding line of the creases is enough to tear the wet web 120.

[0023] The publication of U.S. Pat. No. 5,888,354 discloses a technique enable to inhibit slackness caused by the first longitudinal elongation.

[0024] The technique, a part of which is shown in FIG. 4, has no open draw between a former (not shown) and a first dryer cylinder 202 a, and elongation of a wet web 120 caused during a pressing and dewatering process performed in a press part including a last press 201 a and other elements is taken in by transferring the wet web 120 at different speeds in the press part so as to draw the wet web 120. A felt belt 201 c used for the last press 201 a is stretched to the first dryer cylinder 202 a, and a dryer canvas 202 b runs around a second dryer cylinder 202 d and a turning roll 202 c. With this structure, it is possible to draw the wet web 120 between the first dryer cylinder 202 a and the second dryer cylinder 202 d whereupon the first longitudinal elongation caused at the first dryer cylinder 202 a can be absorbed.

[0025] However, the technique does not release elongation of the wet web 120 in the transfer direction, which elongation is caused at the last press 201 a generating the highest dewatering pressure (nip pressure), immediately downstream of the nip. Microcorrugation is therefore generated on the wet web 120 and the wet web 120, with compressive strain caused by the microcorrugation, is set to the first dryer cylinder 202 a.

[0026] After that, when a paper machine runs, especially at a high speed, elongation enclosed in the form of the compressive strain is released from the wet web 120 between the points C and D, between which neither the felt belt 201 c nor the dryer canvas 202 b press the wet web 120 with respect to the first dryer cylinder 202 a. As a consequence, the wet web 120 is extricated from the outer surface of the first dryer cylinder 202 a between the points C and D, thereby causing the wet web 120 to become largely slack at outer surface of the first dryer cylinder 202 a, as shown by the interrupted line in FIG. 4. The separation point at which the wet web 120 starts disengaging from the outer surface of the first dryer cylinder 202 a between the points C and D varies based on various conditions, such as a centrifugal force acting on the wet web 120, the adhesive force and/or a compression strain amount of the wet web 120, and/or a nip pressure between the first dryer cylinder 202 a and a nip roll 201 b. Slackness due to disengagement of the wet web 120 is unstable in behavior and causes tearing to make stable high-speed working of a paper machine difficult.

[0027] It is conceivable that the speed of the first dryer cylinder 202 a is set to be higher than that of the last press 201 a (in other words, the wet web 120 is drawn (stretched so as not become slack) therebetween), so that the compressive strain generated on the wet web 120 downstream from the last press 201 a and the resultant slackness of the wet web 120 caused between the point C and D are removed. However, in order to set the speed of the first dryer cylinder 202 a higher than that of the felt belt 201 c, the nip between the nip roll 201 b and the first dryer cylinder 202 a has to be released by departing the nip roll 201 b from the first dryer cylinder 202 a and the nip pressure has to be reduced by reducing pressure of the nip roll 201 b applied to the first dryer cylinder 202 a.

[0028] In this case, pressure of the felt belt 201 c applied to the wet web 120 with respect to the first dryer cylinder 202 a becomes inadequate and stable transfer of the wet web 120 to the first dryer cylinder 202 a cannot be realized.

[0029] As mentioned above, even in a no-open-draw paper machine that is able to work at a high speed, wet-web tearing resulting from the first and second longitudinal elongation generated in the dryer part is avoidable, making the operation in paper machine unstable. As a result, high-speed operation cannot be realized in a conventional paper machine.

SUMMARY OF THE INVENTION

[0030] With the foregoing problems in view, the object of the present invention is to provide a paper machine that prevents wet web from going slack in order to stably work at a high speed.

[0031] As a first generic feature, there is provided a paper machine comprising: a plurality of belt mechanisms for transferring a wet web which is formed by a wire part, each of the belt mechanisms having a belt for supporting the wet web and a driving unit for driving the belt together with the wet web; a press part including one or more press units, arranged along a transfer path of the web, for pressing the wet web so that the wet web is dewatered; and a dryer part including a plurality of dryer units, arranged along the transfer path of the wet web, for drying the wet web, which has been dewatered in the press part, by heat; wherein a last press unit, that is the most downstream one of the one or more press units along the transfer path of the wet web in the press part, a first dryer unit, that is the most upstream one of the plural dryer units along the transfer path of the wet web in the dryer part, and a second dryer unit, that is the second upstream one of the plural dryer units along the transfer path of the wet web in the dryer part, are associated with three of the belt mechanisms, respectively, and the driving units of the respective different belt mechanisms, which are associated with the last press unit, the first dryer unit and the second dryer unit, respectively, are individually controlled so that a transfer speed of the wet web along each of the last press unit, the first dryer unit and the second dryer unit is individually set.

[0032] As a first preferable feature, the transfer speed of the wet web along the first dryer unit may be set to be higher than the transfer speed of the wet web along the last press unit; and the transfer speed of the wet web along the second dryer unit may be set to be higher than the transfer speed along the first dryer unit. With these settings, it is possible to prevent the wet web from going slack upstream and downstream of the first dryer unit so that high-speed papermaking can be realized.

[0033] As another preferable feature, the transfer speed of the wet web along the first dryer unit may be set to be higher than the transfer speed of the wet web along the last press unit by a factor equal to or less than 1.04. As an additional preferable feature, the transfer speed of the wet web along the second dryer unit may be higher than the transfer speed of the wet web along the first dryer unit by a factor equal to or less than 1.01.

[0034] These preferable features can effectively prevent the wet web from tearing, respectively or in combination, and can concurrently realize high-speed stable operation of the paper machine.

[0035] As a further preferable feature, the dryer part may have one or more other dryer units other than the first and second dryer units, which other dryer units are arranged downstream of the first and second dryer units; the second dryer unit and one of the other dryer units may be associated with two of the belt mechanisms, respectively; and a transfer speed of the wet web along the last-named one dryer unit may be set to be higher than the transfer speed of the wet web along the second dryer unit by a factor equal to or less than 1.01. As a result, longitudinal elongation caused by heat mainly at the second dryer unit can be absorbed whereupon the wet web is further effectively prevented from going slack.

[0036] Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a side view schematically illustrating the main portion of a paper machine according to a first embodiment of the present invention;

[0038]FIG. 2 is a side view schematically illustrating a part of the paper machine of FIG. 1; and

[0039]FIGS. 3 and 4 are schematic side views individually illustrating configuration from the last press roll to the entrance of a dryer part of a conventional paper machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.

[0041] First Embodiment:

[0042]FIGS. 1 and 2 are side views illustrating a paper machine according to a first embodiment of the present invention: FIG. 1 shows a side view of the paper machine; and FIG. 2, a side view of a part of the paper machine of FIG. 1.

[0043] As shown in FIG. 1, the main portion of the paper press of the present invention includes a wire part (a paper forming process) X, a press part (a dewatering process) Y and a dryer part (a drying process) Z in this order along the direction of transfer of a wet web. The paper machine has no open draw (i.e., no non-supported space at which a wet web is not supported by any part included in the paper machine) between the wire part X and the downstream exit of the press part Y. Although not shown in an accompanying drawing, the paper machine further includes calendering and reeling processes downstream of the dryer part Z in this order. A web dewatered and dried in the press part Y and the dryer part Z passes through the calendering process to smooth the surface thereof and through the reeling process whereupon the web is made into a reeled paper product.

[0044] The wire part X comprises a flow box 1 to flow pulp suspension (a mixture of fibers and dispersant water) serving as a paper material solution, a pair (two sheets) of endless meshes (wires) 2 and 3 that run in synchronization with each other, a suction roll 4 and a wire roll 5 opposing each other upstream of the paper forming process being interposed by the pair of meshes 2 and 3, drainage devices 6 and 7 facing each other and disposed downstream of the suction roll 4 and of the wire roll 5, and a suction roll 8.

[0045] The mesh 2 is supported by the suction roll 4, a wire roll 51 and a number of guide rolls 53 and is driven or guided by each of the rolls 4, 51 and 53 so that the mesh 4 moves. Similarly, the mesh 3 is supported by the wire roll 5, a wire roll 52 and a number of guide rolls 54 and travels by being driven or guided by each of the rolls 5, 52 and 54.

[0046] In the wire part X, the pulp suspension jetted from the flow box 1 is jetted to a space between the pair of meshes 2 and 3 that are to be introduced between the wire roll 5 and the suction roll 4 and is transferred sandwiched between the meshes 2 and 3. During transfer, water contained in the pulp suspension is drained off by the drainage devices 6 and 7 and is sandwiched between the meshes 2 and 3, so that between pulp fibers and dispersant water are separated and a wet web is formed.

[0047] The subsequent stage of the press part Y includes a pair (two sheets) of endless felt belts (belts) 12 and 13 upstream and another pair (two sheets) of endless felt belts (belts) 18 and 19 downstream.

[0048] The felt belt 12 aligned most upstream of the press part Y runs around two suction rolls 11, a press roll 14 and a number of guide rolls 55 so as to form a loop. One of the suction rolls 11 and the press roll 14 has a driving motor (a driving unit) to drive the felt belt 12 together with a wet web as well as with these rolls. The felt belt 12 travels in accordance with driving or guidance of these rolls 11, 14 and 55. With this structure, a wet web is supported by the felt belt 12 during transfer and the combination of the felt belt 12 and these rolls 11, 14 and 55 serves as one of the belt mechanisms of the present embodiment.

[0049] The felt belt 13 opposite to the felt belt 12 runs around a press roll 15, a suction roll 16 and a number of guide rolls 56 into a loop shape. One of the press roll 15 and the suction roll 16 includes a driving motor (a driving unit) so that the felt belt 13 travels in accordance with driving or guidance of these rolls 15, 16 and 56. The combination of the felt belt 13 and these rolls 15, 16 and 56 serves as another one of the belt mechanisms of the present embodiment.

[0050] A wet web on the mesh 3 is transferred downstream in the wire part X and then picked up by the suction roll 11 to move to the felt belt 12 so that the wet web is dewatered by pressure applied by the press rolls 14 and 15 when sandwiched between the felt belts 12 and 13, which are respectively included in the belt mechanisms of the present embodiment, on the transfer path. The press rolls 14 and 15 serve as a press unit of the present embodiment.

[0051] The wet web passed through between the press rolls 14 and 15 is completely moved to the felt belt 13 by a suction force of the suction roll 16 acting on the web at a separation point between the felt belts 12 and 13. The wet web is further picked up by a suction force of a suction roll 17 to be sent to a top felt belt 18, which is arranged downstream in the press part Y.

[0052] The top felt belt 18 runs around the suction roll 17, a press roll 20 and a number of guide rolls 57 so as to form a loop. One of the suction roll 17 and the press roll 20 has a driving motor (a driving unit) so that the top felt belt 18 is driven or guided by these rolls 17, 20 and 57 to move. Facing the top felt belt 18, a bottom felt belt 19 is installed so as to form a loop by running around a press roll 21 and a number of guide rolls 58. The press roll 21 has a driving motor (a driving unit) and the bottom felt belt 19 travels due to driving or guidance of these rolls 21 and 58.

[0053] The combination of the felt belt 18 and the rolls 17, 20 and 57 serves as one of the belt mechanisms of the present embodiment; and similarly, the combination of the felt belt 19 and the rolls 21 and 58, serves as another one of the belt mechanisms.

[0054] The wet web moved to the felt belt 18 is sandwiched between the traveling felt belts 18 and 19 to be transferred downstream and is further dewatered by being pressed by the last press (i.e., a combination of the press rolls 20 and 21). Namely, the pair of the press rolls 20 and 21 function as a press unit of the first embodiment. The wet web passed through between the press rolls 20 and 21 moves onto the felt belt 19 and is further moved to the dryer part Z by a suction pick-up roll 23.

[0055] However, the most downstream press is preferably a shoe press instead of the press rolls 20 and 21. Additionally, the bottom felt belt 19 is preferably substituted with an impervious transfer belt to avoid rewetting (water squeezed from a wet web being absorbed by the wet web again).

[0056] As shown in FIG. 1, the upstream portion of the dryer part Z includes endless canvas (belts) 22 a to 22 c, wet-web turning rolls (hereinafter sometimes simply called turning rolls) 23 a to 23 c, dryer cylinders (dryer units) 25 a to 25 d, and suction boxes 27 a to 27 d. Vapor is supplied to the inside of each of the dryer cylinders 25 a to 25 d from a non-illustrated external vapor source. The wet web 10 transferred from the press part Y is successively pressed onto the outer surface of each dryer cylinder 25 a to 25 d by the canvases 22 a to 22 c and is gradually dried.

[0057] The most upstream canvas 22 a is disposed so as to be opposite to the most upstream dryer cylinder (hereinafter also called the first dryer cylinder) 25 a, serving as the first dryer unit. The canvas 22 a runs around the suction pick-up roll 23 and a number of guide rolls 59 and thereby forms a loop shape. The outer circumference of the canvas 22 a is partially pressed by the first dryer cylinder 25 a. The canvas 22 a is supported by the suction pick-up roll 23, the dryer cylinder 25 a and the guide rolls 59, and the canvas 22 s is driven by the suction pick-up roll 23 or the dryer cylinder 25 a. Where upon, the canvas 23 a travels in accordance with guidance by the guide rolls 59.

[0058] The canvas 22 b downstream of the canvas 22 a is opposite to the second upstream cylinder (hereinafter called the second drier cylinder) 25 b, serving as the second dryer unit, and the third upstream cylinder (hereinafter called the third dryer cylinder) 25 c. The canvas 22 b is supported and driven/guided by the cylinders 25 b and 25 c, the turning rolls 23 a and 23 b and a number of guide rolls 60 to move.

[0059] The canvas 22 a, the dryer cylinder 25 a and the guide rolls 59 function as an integrated form of one of the belt mechanisms and one of the dryer units of the present embodiment. Similarly, the canvas 22 b, the cylinders 25 b and 25 c, turning rolls 23 a and 23 b and the guide rolls 60 make an integrated form having one of the belt mechanisms and one of the dryer units of the present embodiment.

[0060] The canvas 22 c, entry of which only appears in FIG. 1, is arranged so as to be opposite to the cylinder (hereinafter called the fourth dryer cylinder) 25 d fourth upstream in the direction of transfer of a wet web. The canvas 22 c is supported and driven/guided by the fourth dryer cylinder 25 d, turning roll 23 c, and a number of guide rolls 61 to travel.

[0061] Hereinafter, the dryer part Z will now be described in detail with reference to FIG. 2, in which the interrupted line represents the wet web 10 for convenience.

[0062] The wet web 10 being transferred along with the bottom felt belt 19 of the last press unit is picked up by the suction pick-up roll 23 interposed by the canvas 22 a associated with the first dryer cylinder 25 a. After that, the wet web 10 is pressed onto the first dryer cylinder 25 a by the canvas 22 a.

[0063] A wet-web sucking device (in the illustrated example, a suction box for generating a negative pressure) 27 a is installed along a portion of the inside of the loop-shaped canvas 22 a between the suction pick-up roll 23 and the first dryer cylinder 25 a. The suction box 27 sucks up the wet web 10 onto the canvas 22 a in order to avoid disengagement of the wet web 10 from the canvas 22 a while the wet web 10 is transferred from the suction pick-up roll 23 to the first dryer cylinder 25 a.

[0064] The canvas 22 a presses the wet web 10 onto the portion between the points H and K of the outer surface of the first dryer cylinder 25 a (that is, the canvas 22 a is pressed onto the outer surface of the first dryer cylinder 25 a between the points H and K with the wet web 10 interposed therebetween). The canvas 22 a disengaged from the outer surface of the first dryer cylinder 25 a (at the point K) enters a return loop to travel along a stretching unit, a cleaning unit and other unit, which do not appear in the accompanying drawing, and finally returns to the suction pick-up roll 23.

[0065] A guide roll 60 a that is the closest to the first dryer cylinder 25 a among the guide rolls 60 that guide traveling of the second canvas 22 b includes a non-illustrated shifting mechanism so that the guide roll 60 a can move forward and backward with respect to the first dryer cylinder 25 a in the direction shown by the arrow in FIG. 2. Adjustment of the position of the guide roll 60 a adjusts a part of the transfer path of the second canvas 22 b. A part of the transfer path of the second canvas 22 b is set such that the second canvas 22 b is in the closest proximity to or in kiss touch with the outer surface of the first dryer cylinder 25 a at point L. Further, adjustment of the position of the guide roll 60 a adjusts the gap width between the second canvas 22 b and the first dryer cylinder 25 a, and also adjusts the degree of the kiss touch (the nip pressure or the contact amount) of the second canvas 22 b with the first dryer cylinder 25 a. Such adjustment of the position of the guide roll 60 a is carried out in accordance with various conditions, such as type of wet web 10 or transfer speed, such that the wet web 10 does not go slack.

[0066] Here, the point L represents the disengagement point at which the wet web 10 disengages from the first dryer cylinder 25 a if the second canvas 22 b does not press the wet web 10 onto the first dryer cylinder 25 a. Being in kiss touch stands for a substantial point contact between the second canvas 22 b and the first dryer cylinder 25 a so that the nip pressure between the second canvas 22 b and the first dryer cylinder 25 a is substantially equal to zero.

[0067] The outer surface of the first dryer cylinder 25 a is covered with ceramics so that the pressed wet web 10 disengages from the surface with ease. Doctor blades 28 and 29 and an air jet 35 are disposed at a rotational portion (here, the bottom outer surface) of the first dryer cylinder 25 a which portion does not contact with the wet web 10 being transferred. The edges of doctor blades 28 and 29 contact with the outer surface of the first dryer cylinder 25 a allowing the rotation of the first dryer cylinder 25 a, and the air jet 35 for shooting air jets onto the cylinder outer surface. The doctor blade 28 upstream in the cylinder rotation and the air jet 35 disengage the possible wet web 10 sticking on the outer surface of the first dryer cylinder 25 a downstream of the point L from the outer surface of the first dryer cylinder 25 a. Meanwhile, the doctor blade 29 downstream in the cylinder rotation scrapes off the extraneous matters to clean the cylinder outer surface. Keeping the cylinder outer surface clean stabilizes behavior of the wet web 10 when shifting to the next stage (e.g., disengagement of the wet web 10 from the cylinder outer surface at a substantially constant point).

[0068] The second canvas 22 b after passing the point L is pressed onto the outer surface of the second dryer cylinder 25 b through the outer surface of the wet-web turning roll 23 a. After that, the second canvas 22 b passes down stream along the wet-web turning roll 23 b, the third dryer cylinder 25 c, and the guide rolls 60, and further along a stretching unit, a cleaning unit and other unit, which do not appear in the accompanying drawing, and finally returns to the guide roll 60 a.

[0069] A wet-web sucking device (in the illustrated example, a suction box for generating a negative pressure) 27 b is installed along the inside of the loop-shaped second canvas 22 b. The wet-web turning roll 23 a generates negative pressure around the outer surface of the turning roll 23 a so that the wet web 10 and the second canvas 22 b are sucked onto the outer surface of the wet-web turning roll 23 a together whereby the wet web 10 can be stably passed over to the second canvas 22 b from the first canvas 22 a.

[0070] In order to realize stable traveling of the wet web 10 along the outer surface of the first dryer cylinder 25 a between the points K and L, the second canvas 22 b preferably has an air permeability of 1,000-4,000 cc/cm² min@½″AQ (i.e., an air permeability of 1,000-4,000 cm³/cm² min at a differential pressure of 124.5 Pa).

[0071] As mentioned above, the last press consisting of press rolls 20 and 21, the first dryer cylinder 25 a and the second dryer cylinder 25 b are individually associated with upstream belt mechanisms whereupon the felt belts 18 and 19 for the last press, the canvas 22 a for the first dryer cylinder 25 a and the canvas 22 b for the second dryer cylinder 25 b can be set to move at respective different speeds. In other words, it is possible to set individual speeds of the wet web 10 being transferred along the last press and the first and second dryer cylinders 25 a and 25 b, so that the wet web 10 can be drawn respectively between the last press and the first dryer cylinder 25 a and between the first dryer cylinder 25 a and the second dryer cylinder 25 b.

[0072] Here, the canvas 22 a travels along the first dryer cylinder 25 a at a speed V₁, which is set to be higher than the speed V₀ of the last press traveling (i.e., the traveling speed of the top and bottom felt belts 18 and 19) by a factor equal to or less than 1.04 (i.e., V₀<V₁≦1.04V₀), concerning longitudinal elongation (approximately 3%) of the wet web 10 caused at the last press so that the wet web 10 can be drawn between the last press and the first dryer cylinder 25 a. For this purpose, a driving unit able to set the traveling speed of the canvas 22 a in the above range is selected as the driving unit (a motor to rotationally drive the suction pick-up roll 23 or the first dryer cylinder 25 a) to drive the canvas 22 a. A speed V₁ in excess of 1.04 times V₀ causes the wet web 10 to be excessively drawn, lowering the stiffness of the wet web 10. Excessive drawing may be a cause of tearing of the wet web 10 in a downstream dryer cylinder and make it difficult to work the paper machine at a high speed. Therefore the speed V₁ is preferably higher than the speed V₀ by a factor equal to or less than 1.03.

[0073] The speed V₂ of the canvas 22 b traveling along the second and third dryer cylinder 25 b and 25 c is set to be higher than the speed V₁, for example, by a factor equal to or less than 1.01 (i.e., V₁<V₂≦1.01V₁), concerning a possible first longitudinal elongation (equal to or less than 1%, normally 0.5%), which has been referred to as a problem in conventional techniques, so that the wet web 10 can be drawn between the first dryer cylinder 25 a and the second dryer cylinder 25 b. For this purpose, a driving unit able to set the traveling speed of the canvas 22 b in the above range is selected as the driving unit (a motor to rotationally drive one of the wet-web turning rolls 23 a and 23 b and the dryer cylinders 25 b and 25 c) to drive the canvas 22 b.

[0074] Further, in the illustrated example, the fourth dryer cylinder 25 d is associated with a belt mechanism different from that associated with the second and third dryer cylinders 25 b and 25 c, as described above. This can individually set a traveling speed for each of the canvas 22 b (for the second and third dryer cylinders 25 b and 25 c) and the canvas 22 c (for the fourth dryer cylinder 25 d) to move at. In other words, it is possible to set individual speeds of the wet web 10 along both the second and the third dryer cylinders 25 b and 25 c and along the fourth dryer cylinder 25 d, respectively.

[0075] Generally, the temperature of a wet web sharply rises while a wet web is transferred along several drier cylinders upstream in the dryer part Z and then the wet web attains equilibrium at approximately 80° C. After that, the remaining water in the wet web is evaporated at a constant temperature. Assuming that the temperature of the wet web 10 is 20° C. when entering the dryer part Z, the coefficient of expansion of the water is approximately 2.7% if the water temperature rises from 20° C. to 80° C. The degree of expansion differs according to the direction because of orientation of wet-web fibers. On the assumption that the wet web uniformly expands in every direction, the coefficient of expansion is converted into the coefficient of linear expansion of approximately 1%. In other words, the second longitudinal elongation of the wet web 10 is approximately 1%.

[0076] In order to absorb (take in) the second longitudinal elongation of the wet web 10, the speed V₃ of the canvas 22 c traveling along the fourth dryer cylinder 25 d is set to be approximately 1.01 times as high as the speed V₂ of the upstream canvas 22 b. A driving unit able to set the traveling speed V₃ of the canvas 22 c is selected as the driving unit (e.g., a motor for rotationally driving the third drier cylinder 25 c) for driving the canvas 22 c.

[0077] In the dryer part Z, since the wet web 10 generates a second longitudinal elongation until the wet web 10 attains the equilibrium at 80%, each dryer cylinder is preferably associated with a dedicated canvas loop and a dedicated driving source to drive the dedicated canvas loop (that is, an individual belt mechanism is preferably provided for a dedicated one of the dryer cylinders). However, as described above, the rising in temperature (i.e., the second longitudinal elongation) of the wet web 10 is high at the entry of the dryer part Z but the temperature of the wet web scarcely rises in the downstream dryer part Z. A result of the Inventor's experimental use of a pilot machine reveals that it is possible to prevent the wet web 10 from going slack by installing at least one belt mechanism to drive a drier cylinder disposed downstream of the second dryer cylinder 25 b separately from the belt mechanism to drive the second dryer cylinder 25 b such that an elongation generated in at least the second dryer cylinder 25 b is absorbed.

[0078] In the paper machine of the first embodiment having the above mentioned structure, when a wet-web elongation generated at the last press is absorbed by setting the transfer speed V₁ of the wet web 120 transferred together with the first canvas 22 a to be higher than the transfer speed V₀ of the wet web 120 transferred along the last press (V₀<V₁), the wet web 10 generates a first longitudinal elongation similar to a conventional technique, which elongation can however be absorbed by setting the transfer speed V₂ of the wet web 120 transferred together with the second canvas 22 b to be higher than the transfer speed V₁ of the wet web 120 transferred together with the first canvas 22 a (V₁<V₂).

[0079] The wet web 10 does therefore not go slack on the canvas 22 b when being transferred to the second dryer cylinder 25 b so that the substantial entire portion of the wet web 10 being transferred together with the second canvas 22 b is supported by the second canvas 22 b, the wet-web turning roll 23 a and the second dryer cylinder 25 b. As a result, it is possible to almost-completely avoid slackness of the wet web 10, which is a cause of creases resulting in stress concentration, so that tearing of the wet web 10 can be inhibited.

[0080] Here, the third canvas 22 c, which is different from the second canvas 22 b running around the second and third dryer cylinder 25 b and 25 c, runs around the fourth cylinder 25 d whereby the traveling speed of the second canvas 22 b can be set separately from the traveling speed of the third canvas 22 c.

[0081] While the wet web 10 is gradually heated and dried during proceeding in the dryer part Z, the above-mentioned second longitudinal elongation is generated at and downstream of the second dryer cylinder 25 b (especially, at the second dryer cylinder 25 b). The second longitudinal elongation can be absorbed by setting the speed of the third canvas 22 c to be higher than the speed of the second canvas 22 b, so that the wet web 10 can be drawn between the third dryer cylinder 25 c and the wet-web turning roll 23 c. As a result, it is possible to prevent the wet web 10 from going slack on the third canvas 22 c and further from being torn.

[0082] With this structure and driving manner, the wet web 10 can be stably transferred in the dryer part Z even at a high speed, so that frequency of tearing is greatly reduced as compared with a conventional technique. As a result, it is possible to run the paper machine at as high a wet-web transfer speed as 2,000 m/min, for example.

[0083] Further, the paper machine of the present invention should be by no means limited to the foregoing embodiment, and various changes or modifications may be suggested without departing from the gist of the invention.

[0084] A belt mechanism is shared by the second dryer cylinder 25 b and the third dryer cylinder 25 c in the first embodiment. Alternatively, the second dryer cylinder 25 b and the third dryer cylinder 25 c may be associated with different belt mechanisms and the wet web 10 may be drawn between these dryer cylinders 25 b and 25 c. With this alternative configuration, it is possible to early absorb a second longitudinal elongation generated on the wet web 10 at the second dryer cylinder 25 b.

[0085] The speeds V₀, V₁, V₂ and V₃ of the felt belt 18 and 19, and the canvas 22 a, 22 b and 22 c should be by no means limited to the ranges described above, and alternatively can be set to appropriate values so as to absorb elongations of the wet web 10. 

What is claimed is:
 1. A paper machine comprising: a plurality of belt mechanisms for transferring a wet web which is formed by a wire part, each of said belt mechanisms having a belt for supporting the wet web and a driving unit for driving said belt together with the wet web; a press part including one or more press units, arranged along a transfer path of the web, for pressing the wet web so that the wet web is dewatered; and a dryer part including a plurality of dryer units, arranged along the transfer path of the wet web, for drying the wet web, which has been dewatered in said press part, by heat; wherein a last press unit, that is the most downstream one of the one or more press units along the transfer path of the wet web in said press part, a first dryer unit, that is the most upstream one of the plural dryer units along the transfer path of the wet web in said dryer part, and a second dryer unit, that is the second upstream one of the plural dryer units along the transfer path of the wet web in said dryer part, are associated with three of said belt mechanisms, respectively, and said driving units of the last-named three belt mechanisms, which are associated with said last press unit, said first dryer unit and said second dryer unit, respectively, are individually controlled so that a transfer speed of the wet web along each of said last press unit, said first dryer unit and said second dryer unit is individually set.
 2. Paper machine according to claim 1, wherein: the transfer speed of the wet web along said first dryer unit is set to be higher than the transfer speed of the wet web along said last press unit; and the transfer speed of the wet web along said second dryer unit is set to be higher than the transfer speed along said first dryer unit.
 3. A paper machine according to claim 1, wherein the transfer speed of the wet web along said first dryer unit is set to be higher than the transfer speed of the wet web along said last press unit by a factor equal to or less than 1.04.
 4. A paper machine according to claim 2, wherein the transfer speed of the wet web along said first dryer unit is set to be higher than the transfer speed of the wet web along said last press unit by a factor equal to or less than 1.04.
 5. A paper machine according to claim 1, wherein the transfer speed of the wet web along said second dryer unit is higher than the transfer speed of the wet web along said first dryer unit by a factor equal to or less than 1.01.
 6. A paper machine according to claim 2, wherein the transfer speed of the wet web along said second dryer unit is higher than the transfer speed of the wet web along said first dryer unit by a factor equal to or less than 1.01.
 7. A paper machine according to claim 3, wherein the transfer speed of the wet web along said second dryer unit is higher than the transfer speed of the wet web along said first dryer unit by a factor equal to or less than 1.01.
 8. A paper machine according to claim 4, wherein the transfer speed of the wet web along said second dryer unit is higher than the transfer speed of the wet web along said first dryer unit by a factor equal to or less than 1.01.
 9. A paper machine according to claim 1, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 10. A paper machine according to claim 2, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 11. A paper machine according to claim 3, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 12. A paper machine according to claim 4, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 13. A paper machine according to claim 5, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 14. A paper machine according to claim 6, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 15. A paper machine according to claim 7, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01.
 16. A paper machine according to claim 8, wherein: said dryer part has one or more other dryer units other than said first and second dryer units, which other dryer units are arranged downstream of said first and second dryer units; said second dryer unit and one of said other dryer units are associated with two of said belt mechanisms, respectively; and a transfer speed of the wet web along said last-named one dryer unit is set to be higher than the transfer speed of the wet web along said second dryer unit by a factor equal to or less than 1.01. 